1. Field of the Invention
The present invention relates to an improvement in an extendible nozzle exit cone for rocket motors that are designed for operation at vacuum or near vacuum conditions.
2. Description of the Prior Art
Deep space ballistic missile systems or satellite probes require high performance, low weight, and highly packageable primary propulsion systems. Excluding propellant tanks, the largest component of the propulsion system is the rocket motor exhaust nozzle. The rocket motor nozzle takes up a large amount of valuable space relative to its mass.
The exit cone of a conventional nozzle for rocket motors is designed for optimum performance at the median altitude of the intended trajectory. One of the functions of the exit cone is to provide an inclined surface against which the expanding exhaust plume of the rocket motor can bear, thereby to provide some of the forward thrust of the rocket motor. The exhaust plume grows larger with increasing altitude of the rocket motor because of the lower pressures of the ambient atmosphere at the higher altitudes. At low altitudes, the exhaust plume is too small for the available surface of the exit cone. As a result, a partial vacuum tends to form on the inside edges of the exit cone, thereby creating an atmospheric drag on the rocket. At high altitudes, the exhaust plume is too large for the exit cone so that much of the potential energy is unused. A rocket motor nozzle that is sufficiently large to make full use of the expanding exhaust gases of a rocket motor in the low pressures existing at high altitudes would normally occupy an inordinately large proportion of the available storage space in silos, submarines, and between stages of a multiple-stage missile.
Various proposals have been made in the prior art to provide a large expansion ratio nozzle that can be stowed in a configuration of reduced length and thereby made to fit in a minimal space, and that can be extended to a configuration suitable for high altitude operation after motor ignition and lift off. These have included the use of:
(a) a rocket motor nozzle extension cone or skirt that is folded in a single layer inwardly and forwardly of the rocket motor nozzle when in a stowed position, and is actuated, that is, deployed to a fully extended position by foward-to-aft flow of rocket motor gas, as disclosed in U.S. Pat. No. 3,358,933 to J. H. Altseimer, and in U.S. Pat. No. 4,272,956 to G. C. Lamere et al.; PA1 (b) an inflatable rocket motor extension cone or skirt that is folded forwardly of, that is back around the rocket motor nozzle, exteriorly thereof, and later inflated by rocket motor gas to achieve a desired frusto conical shape aft of the rocket motor nozzle, as in U.S. Pat. No. 3,596,465 to T. O. Paine et al.; PA1 (c) a rocket motor extension cone or skirt that is folded forwardly of, that is, back around the motor nozzle, exteriorly thereof, and including a cover assembly attached to the aft or exit end of the skirt that seals the extension cone, and upon motor operation, seals the gas pressure therein sufficient to cause the skirt to unroll into its extended configuration, as in U.S. Pat. No. 3,711,027 to L. F. Carey and U.S. Pat. No. 3,784,109 to J. W. Dueringer; PA1 (d) a rocket motor extension cone that is folded forwardly of, that is, back around the motor nozzle, exteriorly thereof, and including a plurality of mechanical actuators for causing the skirt to unroll into its extended configuration, as in U.S. Pat. No. 3,346,186 to D. L. Fulton et al., U.S. Pat. Nos. 4,125,224, 4,162,040, 4,184,238 and 4,387,564 to L. F. Carey, U.S. Pat. No. 4,213,566 to L. E. Miltenberger, and U.S. Pat. Nos. 4,383,407 and 4,489,889 to F. S. Inman; PA1 (e) a rocket motor extension cone as described in item (d) further including a skirt attached to the aft end of the extension cone that is flared toward the interior of the extension cone when the latter is in its stowed position and is actuated into its extended configuration by a forward-to-aft flow of rocket motor gas, as in U.S. Pat. Nos. 4,125,244, 4,162,040, 4,184,238, and 4,387,564 to L. F. Carey.
There are problems with the prior art proposals for providing high performance over the entire range of intended trajectory. They are deficient in meeting the large expansion ratio, that is, the ratio of the area at the exit plane of the extendible nozzle cone to the area of the nozzle throat, required for high altitude rocket flight while still remaining within length, weight and economic limiting constraints.
Thus, with respect to the prior art mentioned in item (a) above, which prior art is schematically represented by FIG. 1 of the drawings, it is evident that since the length L.sub.s along the straight meridian of the extended cone portion 1 of a rigid cone 2 cannot exceed the radius R.sub.f, the exit opening of cone 2, the length L.sub.a along the longitudinal axis 3 of the cone extension 1 must be smaller than L.sub.s. This seriously limits the expansion ratio that is obtainable by the prior art of item (a) since the length L.sub.s must be short enough to fit within cone 2 in the stowed condition.
The prior art of item (b) involves the use of an inflatable extension cone consisting of two woven stainless steel interconnected panels and including a manifold connection for dumping rocket motor gas between the panels for inflating the cone. The double panel and manifold connection both add undesirably to the weight, bulk and complexity of the construction.
With respect to the prior art of item (c), the requirement for the cover assembly that is attached to the exit end of the extension skirt adds undesirably to the weight of the extension cone, the transverse load thereon, and the complexity of the construction.
The prior art of items (d) and (e) involve the use of mechanical actuators such as pneumatic cylinders that add undesirably to the weight, bulk and complexity of the construction.
In my copending application for patent bearing Ser. No. 762,472 filed Aug. 15, 1985, there is disclosed an extendible rocket motor nozzle exit cone that is folded inwardly of itself, in multiple layers from the aft end, to provide a very large area ratio at the exit plane to be stowed in a much reduced envelope in comparison to the aforementioned prior art. As a consequence of the reduced length of the forward portion of the cone, and because the stowed aft portion maintains the shape at the interface, the cone, in the stowed condition thereof, is quite rigid.
The present invention is concerned particularly, in a first embodiment, with a form of extendible rocket motor exit cone in which an aft portion of the membrane of the nozzle extension is stowed in a single layer of fluted form, being folded inwardly of itself and contained between two planes perpendicular to the cone axis, and in which the forward portion of the membrane of the cone is extensively folded. In the first embodiment, the maximum overall reduction in length is less than is achievable with the extendible rocket motor exit cone of my aforesaid copending application for patent, but the forward portion of the cone is more rigid in the stowed condition. In a second embodiment, a somewhat smaller reduction of envelope is obtained than with the extendible rocket motor exit cone of my copending application, but the stowed configuration of the forward portion of the membrane is stiffer by virtue of the folds.